The decision of whether or not to apply post-fire hillslope
erosion mitigation treatments, and if so, where these treatments
are most needed, is a multi-step process. Land managers must
assess the risk of damaging runoff and sediment delivery events
occurring on the unrecovered burned hillslope. We developed the
Erosion Risk Management Tool (ERMiT) to address this need. ERMiT
is a web-based application that uses the Water Erosion Prediction
Project (WEPP) technology to estimate sediment delivery, in probabilistic
terms, on burned and recovering forest, range, and chaparral
lands with and without the application of mitigation treatments.
User inputs are processed by ERMiT to combine rain event variability
with spatial and temporal variability of soil burn severity and
soil properties, which are then used as WEPP input parameter
values. Based on 20 to 40 individual WEPP runs, ERMiT produces
a distribution of single sediment delivery rates with a probability
of occurrence for each of five postfire years. In addition, sediment
delivery rate distributions are generated for postfire hillslopes
that have been treated with seeding, straw mulch, and erosion
barriers such as contour-felled logs or straw wattles. Using
postfire sediment data from 21 small instrumented watersheds
(< 14 ha), we compared each storm's measured sediment delivery
to the ERMiT-predicted delivery. Observed delivery rates were
within the predicted range of values 77 percent of the time,
with 14 percent of the observed values being greater than the
estimated range, and 9 percent being less than the predicted
range. Most of the under predictions were associated with studies
in the Colorado Front Range. The ERMiT tool tended to over predict
sediment delivery in the Northern Rockies and in California.
Only 3 percent of the observed delivery events were associated
with snow melt processes, whereas 36 percent of the predicted
values were influenced by snow melt. Based on these results,
we are considering improvements such as incorporating erodibility
values for more forest soil types, adjusting the weather characteristics
in the climate generator, and reducing the occurrence of snow
melt erosion response. Postfire assessment teams are actively
using the ERMiT model for making hillslope mitigation treatment
decisions based on the probability of damaging sediment delivery
occurring after a wildfire.

Keywords: ERMiT, wildfire, erosion, WEPP, postfire,
validation

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